1. Field of the Invention
The invention relates to a method of connecting a first workpiece to a second workpiece, for example, a method of connecting an anode rod to an end plate of a rotor sleeve in a rotary anode X-ray tube.
2. Description of the Related Art
In addition to a device for generating an electron beam, rotary anode X-ray tubes include essential components such as a rotor sleeve, one end of which is closed by an end plate, as well as an anode rod (anode shaft) which is connected thereto and supports an anode disc (rotary anode).
Said components are made of very different materials. Whereas the rotor sleeve is usually made of copper, the end plate consists of an iron nickel cobalt alloy (FeNiCo) or nickel 42, and the anode rod often is made of a tungsten zirconium molybdenum alloy (TZM). Generally speaking, the anode rod is connected to the end plate by soldering while using a high-temperature solder. Said materials, however, have very different melting points so that the soldered joint is often faulty due to solder gaps, bonding faults etc. However, because the rotor and notably the anode rod may be subject to severe mechanical loads due to shocks or unbalance of the rotary system, particularly this joint must satisfy very severe requirements.
The problem is accentuated by the fact that the anode rod should also act as a thermal barrier in order to avoid an increased heat flow from the anode disc to the rotor sleeve and the rotary system situated therebelow, and hence the cross-sectional surface area of this rod should be as small as possible.
Because of the foregoing, the rejection rate due to soldered joints that are faulty to even a small extent only is comparatively high and the strength that can be achieved by way of this joint is comparatively low.
DE-OS 29 15 418 discloses a method of connecting workpieces consisting of materials having very different melting points; according to this method the connection surfaces of the workpieces are mechanically joined by friction welding after which they are permanently bonded by diffusion welding. This method, however, has a significant drawback in that it is rather intricate because of the two welding operations and that the quality and strength of the joint often are not reproducible.
Therefore, it is a principal object of the invention to provide a method of connecting a first workpiece to a second workpiece which enables these workpieces, for example an anode rod and an end plate in a rotary anode X-ray tube, to be connected to one another in a reproducible manner and with a high strength.
This object is achieved by means of a method in accordance with the invention which includes the following steps:
friction welding the first workpiece to the second workpiece;
reducing the cross-section of at least one of the two workpieces in a segment which lies outside a connection zone in which the friction weld is situated, that is, in such a manner that the strength of the connection zone is at least slightly greater than that of the segment of reduced cross-section.
Thus, the resistance to fracture is no longer imposed by the friction weld and any, hardly predictable material influencing that is caused thereby, but exclusively by the strength of the segment of reduced cross-section. Because this strength is dependent exclusively on the type of material used and on the cross-section per se, the strength of the joint achieved in accordance with the invention can be reliably reproduced.
The reduction of the cross-section at the same time reduces the thermal conductivity; this is particularly advantageous for the described application in rotary anode X-ray tubes. The cross-section is made only as large as absolutely necessary for achieving the strength required for reliable operation. The reduction of the cross-section thus advantageously combines two different functions.
A further advantage resides in the fact that the method is also suitable for the connection of workpieces of materials having very different melting points, because any influencing of the strength of the friction weld due to said differences can be taken into account by a correspondingly greater reduction of the cross-section relative to that of the connecting zone so as to achieve said object nevertheless.
Depending on the type, shape and dimensions of the workpieces to be connected, the method may also be executed in such a manner that first the cross-section of one of the workpieces is reduced and subsequently friction welding is performed.
The dependent claims relate to advantageous further embodiments of the invention.
The further embodiment in conformity with claim 2 is useful for most materials, because the heat caused by friction welding could weaken the material in the areas of the workpieces that adjoin the welded surfaces.
The embodiment disclosed in claim 3 is intended to achieve a further reduction of the thermal conductivity.
Finally, claim 4 discloses a particularly attractive application for the method in accordance with the invention.